http://wtc.nist.gov/pubs/factsheets/faqs_8_2006.htm6. How could the WTC towers collapse in only 11 seconds (WTC 1) and 9 seconds (WTC 2)—speeds that approximate that of a ball dropped from similar height in a vacuum (with no air resistance)?
NIST estimated the elapsed times for the first exterior panels to strike the ground after the collapse initiated in each of the towers to be approximately 11 seconds for WTC 1 and approximately 9 seconds for WTC 2. These elapsed times were based on: (1) precise timing of the initiation of collapse from video evidence, and (2) ground motion (seismic) signals recorded at Palisades, N.Y., that also were precisely time-calibrated for wave transmission times from lower Manhattan (see NCSTAR 1-5A).
I just reread the above and it’s a bit more ambiguous than I initially thought. The question (heading) refers to the towers, whilst the answer refers to “the first exterior panels.”
I haven’t yet heard a standard explanation regarding the vertical distribution of debris, but my take is as follows:
We know from the resultant debris field that large chunks of exterior columns travelled over 100m laterally. So at the moment these chunks left the tower we know they must have had sufficient lateral velocity to travel that distance. One moment they are attached to the tower, but the next moment they are ejected sideways; i.e. they were accelerated by a short but intense force.
Now suppose the direction of that short but intense force is fairly random. Then one would expect to see a randomized distribution of initial velocities including upward, lateral and downward directions. This would result in a vertical distribution of the debris, with some chunks clearly moving ahead of the main field.
Or in short, the thing exploded.
As a general point, it's probably not wise to pin too much on any one anomaly; e.g. freefall spead. There's so many anomalies, lies and unanswered questions that the sheer weight of them is likely to be what wins through.